这里为什么用完成时.the exam must have been very difficult.her husband had been killed in a car crash.完成时我记得是强调对现在或者过去的影响,但是这两个句子里体现不出来啊,帮我分析下这两个句子,

Question

这里为什么用完成时.
the exam must have been very difficult.
her husband had been killed in a car crash.
完成时我记得是强调对现在或者过去的影响,但是这两个句子里体现不出来啊,帮我分析下这两个句子,

Answer 1

must have done/been 表示的是对发生在过去的事情的一个 肯定猜测,
比如,他一定是把蛋糕都吃了.He must have finished all of the cakes.
否定猜测是can't have done/been,比如 他不可能把这些蛋糕都吃了 He can't have eaten all of the cakes!
跟这个类似的还有,should (not) have done/been,might (not) have done/been need (not) have done/been,只有must 不能直接说must not have done/been 用can not have done/been
第二句,我回答过其他关于过去完成时的用法,过去完成时有时候特指时间点的位置,表示比过去式早一个点,比如我给你改成," When I broke into the house,her husband had been killed." 当我破门而入了,她丈夫已经被杀了.表示在进去之前就死掉了.有先后顺序.也跟你说的"对过去的影响" 不冲突啊,如果他在我进门后死的,表示我这个警察办事不利,但是我冲进去前就死了,那是没办法的事.